Doc:Add escape in the SQL statement in the flink engine document

[hililiwei]

about #3075
the default keyword is used in the SQL statement in the Flink document, but the example does not add escape, so the statement cannot be run directly.
such as：

CREATE TABLE hive_catalog.default.sample (
    id BIGINT COMMENT 'unique id',
    data STRING
);

An error will be reported when the statement is executed：

[ERROR] Could not execute SQL statement. Reason:
org.apache.flink.sql.parser.impl.ParseException: Encountered ". default" at line 1, column 27.

modify the SQL：

CREATE TABLE hive_catalog.`default`.sample (
    id BIGINT COMMENT 'unique id',
    data STRING
);

the above statements can be executed normally.

[openinx]

Thanks @hililiwei to address this issue, I think we can quote all the identifiers (include catalog identifier, database name, table name) , that looks more unified.

[hililiwei]

Thanks.Done.

[openinx]

Get this merged, Thanks @hililiwei for contributing !